Most electronic and computer systems require DC voltage for their operation. A typical power supply for such systems converts AC line current to a regulated DC voltage. A "switch mode" type power supply first converts AC to an unregulated DC which is then controllably pulsed to a regulated DC output. For example, conversion of AC to an unregulated DC may be achieved by rectifying the AC and storing the non-regulated DC voltage in bulk capacitors. Current from the bulk capacitors is then periodically switched or pulsed across a primary winding of a transformer. The pulsating D.C. waveform is received by a secondary winding of the transformer, rectified, regulated, and stored in capacitors which provide the primary DC output voltage of the power supply.
"Regulation" of the D.C. output voltage refers to limiting changes or swings in the output voltage level. This is particularly important in many applications where small deviations from the design voltage level can affect the operation of the equipment being supported by such power supply. A voltage regulator which has been used successfully with switch mode power supplies is a magnetic amplifier (mag amp). A mag amp uses one or more saturable reactors either alone or in combination with other components to control power gain. A saturable reactor is a coil with a magnetic core whose reactance may be controlled by applying a voltage to control the blocking capability of the core. As the blocking capability of the core changes, the pulse width of voltage pulses passing therethrough will be changed. For example, if the output voltage were to increase, increasing the blocking capability of the saturable reactor decreases the pulse width of the voltage pulse passing therethrough, thereby decreasing the output voltage. Similarly, small decreases in the output voltage can be overcome by decreasing the blocking capability of the core. A conventional way to control the blocking capability of the core is to develop an error signal from changes in the output voltage and apply the error signal to the control electrode of a transistor. In response, the transistor controls the application of a reset voltage which changes the blocking capability of the core.
In addition to regulating the output voltage by monitoring the output voltage, it is also known to control the output voltage by monitoring both the output voltage and current. Such current mode regulation involves sensing the D.C. output current and developing a voltage proportional to the output current. One technique for sensing the output current involves passing the current through a resistor and detecting the voltage across the resistor. This has the disadvantage of dissipating power, a problem which increases as the current increases.
It is known to use a detection circuit with a current transformer for detection of the discontinuous unregulated input current The secondary winding of the current transformer is connected to a resistor or parallel resistor and capacitor. The voltage developed on the resistor is provided to a differential amplifier, the output of which terminates unregulated input pulses to the power supply when the developed voltage exceeds a predetermined value. Such detection circuits provide a current limit but do not closely regulate the output current.